Swivel tanker floating storage system

ABSTRACT

The specification discloses a fluid storage system which includes a floating storage vessel having a swivel assembly mounted in its bow structure. The stationary core of the swivel assembly is moored to the water bottom by means of anchor lines extending through hawse tubes, while the storage structure is free to pivot about the stationary core in response to the combined forces of wind and water. Mounted centrally in a hollow portion of the stationary core of the swivel assembly is a liquid-gas separator unit. The liquid and gas outputs of the separator unit pass through swivel joints to the storage tanks of the floating structure. The separator unit comprises a main pressure vessel and a surge vessel which absorbs shock loads from the riser flowlines.

United States Patent [721 Inventors Viutila Bratianu New York, N.Y.;Lewis A. Rupp, Wellesley Hilk, Mass. [211 Appl. No. 775,351 [22] FiledNov. 13, 1968 [45] Patented July 6, I971 [73] Assignee Mobil OilCorporation [54] SWlVEL TANKER FLOATING STORAGE SYSTEM 1 Claim, 2Drawing Figs.

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[56] References Cited UNITED STATES PATENTS 3.407,?68 10/1968 Graham 9/83,472,032 10/1969 Howard 9/8 Primary Examiner-Milt0n Buchler AssistantExaminer-Steven W. Weinrieb Attorneys-William J. Scherback, Frederick E.Dumoulin, Paul Van Slyke, Andrew L. Gaboriault and Sidney A. JohnsonABSTRACT: The specification discloses a fluid storage system whichincludes a floating storage vessel having a swivel assembly mounted inits bow structure. The stationary core of the swivel assembly is mooredto the water bottom by means of anchor lines extending through hawsetubes, while the storage structure is free to pivot about the stationarycore in response to the combined forces of wind and water. Mountedcentrally in a hollow portion of the stationary core of the swivelassembly is a liquid-gas separator unit. The liquid and gas outputs ofthe separator unit pass through swivel joints to the storage tanks ofthe floating structure. The separator unit comprises a main pressurevessel and a surge vessel which absorbs shock loads from the riserflowlines.

a O I PATENTEDJUL BIB?! 3.590.407

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PATENTEDJUL sum v 3590,40?

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SWIVEL TANKER FLOATING STORAGE SYSTEM FIELD or THE INVENTION Theinvention described herein relates generally to the production andstorage of fluid minerals, such as oil and gas, underlying a body ofwater.

SUMMARY OF THE INVENTION The present invention provides a novel andimproved system which:

I. Enables economical and efficient storage of fluid minerals underlyinga body of water to wind a surface storage location using a minimum ofparts and equipment;

2. Permits a floating storage structure to pivot about a mooring pointin response to wind and water forces; and

3. Eliminates high pressure seals in swivel joints in the productionflowline.

In accordance with the present invention there is provided a systemincluding a floating storage structure having a shipshaped hull and aswivel assembly mounted in the bow structure along the longitudinalaxis. The swivel assembly includes a stationary core which is journaledin a vertical well in the floating storage structure. In use, thestorage structure is positioned over a submarine production site and thestationary core of the swivel assembly is moored by mooring lines to thewater bottom. A flowline including a flexible connection extends frombottom gathering points for the fluid minerals to the stationary core ofthe swivel assembly. The stationary core includes a separator unit fordividing the fluid minerals into liquid and gaseous components andreducing each component to atmospheric pressure. A swivel joint iscoupled to the top of the stationary core and is in communication withthe liquid outlet of the separator unit. The outlet of the-swivel jointis connected by a flowline to the liquid storage tanks in the floatingstorage structure. The gas component from the separator unit may beflared by means of a flare stack mounted atop the stationary core orpassed through a separate swivel joint to pipelines or otherdestinations.

Since the stationary core of the swivel assembly is moored by the anchorlines to the bottom, the floating storage structure is free to pivotabout the stationary core in response to wind and water forces in themanner of a weather vane. Since the liquid component is reduced toatmospheric pressure in the swivel assembly, the swivel joint may be ofless expensive design and tends to be more trouble-free than if nopreswivel joint separation had been performed. I

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a schematic illustration of one form OF the present inventionused in conjunction with an underwater satellite gathering system; and

FIG. 2 is a side elevational view partially broken away of one form ofthe swivel assembly and separator for use in the system of FIG. 1.

DETAILED DESCRIPTION OF FIG. 1

Referring to FIG. I, a production system is shown utilizing a number ofsubmarine satellite gathering stations 10, each having a series of wellsdrilled through standing conductor pipe elements 12 of a template ring14 resting on the marine bottom 15. The wells are all completed withsubsea wellheads 16 mounted atop the conductor pipes 12 and are in turncapped with connector units 18 for directing the produced fluids fromthe wellheads into the interior of the satellite gathering stations 10.The apparatus and method of installation of such subsea satellitegathering stations are described in the copending application Ser. No.740,520, filed June 27, I968, to William A. TAlley. In, and entitledSubsea Production Station.

The output flow of all of the satellite gathering stations 10 isconnected through shipping lines 22 to a circular manifold 24, locatedon the marine bottom 15 concentric with a tether pipe 26. Producedfluids are transported through the tether pipe 26 from the marine bottom15 to the upper end of pipe 26 several hundred feet below the watersurface. A flexible line 30 extends from the upper end of the tetherpipe 26 to a floating storage structure 32 having a ship-shaped hull.Floating storage structure 32 includes a swivel assembly 38 mounted inits bow structure. Swivel assembly 38 comprises a stationary core 38ajournaled in a vertical well 38b extending through structure 32. Thefloating structure 32 is moored over tether pipe 26 by means of mooringlines 34 extending between anchor points 36 on the marine bottom 15 andstationary core 38a. A transport tanker 40 is illustrated as tiedalongside storage structure 32 while stored fluid is offloaded ontotanker 40.

The swivel assembly 38 is located along the longitudinal centerline ofstorage structure 32, preferably in the forward 20 percent of the lengthof structure 32. Stationary core 38a is shown schematically as beingmounted on bearings 39 so that it is freely rotatable within verticalwell 38b.

Stationary core 38a, being moored by means of anchor lines 34, remainsfixed while the floating storage structure 32 may pivot in the manner ofa weather vane. In this manner, floating storage structure 32 can alwaysbe pointed into the direction of wind and waves.

' A rotatable propulsion unit may be mounted in the stern of structure32 to aid in controlling the position of structure 32.

Mounted on the stationary core 38a is a separator unit 41 for separatingthe production flow into its gaseous and liquid components. An oilswivel joint 43 is shown mounted atop the swivel assembly 38 and isconnected to the liquid outlet of separator unit 41. The outlet pipe 43aof swivel joint 43 is connected by means of a storage line 43b to thestorage tanks located in storage structure 32.

By having a liquid-gas separator 41 mounted in the swivel assembly 38,only low pressure fluids must be directed through the inherently morevulnerable swivel joint 43,

thereby lessening the possibility of leakage and/or permitting the useof less expensive swivels. The high pressure fluids from underwater needpass through only nonswivel, high pressure connections to the separatorunit 41.

A platform 49, having an above-surface deck, is shown supported on ashallower portion of marine bottom 15 where the water isnot deep enoughto justify use of a satellite gathering system. Platform 49 is alsolinked to storage structure 32 by means of a flowline '22interconnecting gathering equipment on platform 49 and the manifold 24at the base of tether pipe 26. The use of the floating storage structure32 in conjunction with platform 49 permits the platform to beconsiderably lighter construction than if a storage facility were builtin to platform 49. In addition, the elimination of the offloadingfacility from platform 49 reduces the loads for which the platform mustbe designed.

DETAILED DESCRIPTION OF FIG. 2

FIG. 2 illustrates a specific design for the swivel assembly 38 andseparator 4 of FIG. 1 which is disclosed in the copending applicationSer. No. 775,241, filed Nov. I3, 1968, to William A. Talley, Jr., andentitled Swivel and Separator Assembly for Floating Vessel.

Referring to FIG. 2, the swivel assembly 38 comprises a stationary core380 which is rotatably mounted within vertical well 38b. Stationary core38a includes an outer frame 48 having upper and lower axial rollerbearing sets 50 and 52, respectively, and a set of radial thrustbearings 54. The axial roller bearings 50 and 52 bear on circumferentialwear plates 56 and 58, respectively, which are mounted in recesses inthe wall of vertical well 38b. The set of thrust bearings 54 bears on aring wear plate 60 fixedly mounted on a horizontal ledge in the upperportion of vertical well 38b.

The entire stationary core 38a can be removed from vertical well 38b byaderrick or crane, after the various fluid and anchor lines have beendisconnected, by merely lifting the assembly straight up out of thevertical well.

Hawse tubes 62 displaced at angular intervals extend vertically throughthe central bore in stationary core 38a. A number of mooring chains 64,anchored in the marine bottom, extends up through the body of water intothe hawse tubes 62 from which they emerge at the upper end of stationarycore 38a and are connected to padeyes 66 on outer frame 48.

Mounted within the central bore of stationary core 38a, above a pair ofwalkaround platforms 68 an within the hawse tubes 62, is a lowtemperature separator 70. Separator 70 is securely fixed withinstationary core 38a by bolted flange connection 72 mounted between theouter face of main pressure vessel 74 of separator 70 and the hawsetubes 62. The separator equipment within the main pressure vessel 74 issimilar to that found in many other low temperature separator units andincludes inlet deflectors, stilling baffles, a two-phase float valve,and a mist extractor.

Mounted on the outer face of the main pressure vessel 74 is an auxiliarysurge vessel 76. The inlet of surge vessel 76 is connected by a rigidinlet pipe 78 with the upper end of flexible line 30. The oil and gasmixture raising through the tether pipe 26 and .the flexible line 30often has discrete volumes of gas followed by volumes of oil. Suchdiscontinuous fluid flow produces extreme shock loads when arriving atthe terminus of its transportation path. The purpose of the surge vessel76 is to accept such shock loads that could damage the main pressurevessel 74. While the surge vessel 76 includes no special equipment, itdoes tend to cause a substantial vertical separation of the gas from theoil. For this reason the gas line 82 is provided from the upper end ofsurge vessel 76 for conducting mostly gas into the main pressure vessel74 below its mist extractor. An outlet pipe 84 connects the lower end ofsurge vessel 76 with an inlet port midway up the main pressure vessel74.

Final separation of the oil-gas mixture occurs within the man pressurevessel 74, the gas being removed from the main pressure vessel 74through a central upstanding pipe 86 connected to a gas outlet line 88by a right angle swivel joint 90.

The oil separated out of the produced fluid is removed from the mainpressure vessel 74 through a swivel joint comprising a concentric pipe92 enclosing the lower end of the upstanding pipe 86 and an outerconcentric swivel yoke 94 rotatably mounted thereon. The outlet of yoke94 is connected to a storage manifold 96 by an oil outlet 98. The oildirected to the manifold 96 is transported down into the storagecompartments 44. The gas flowing in line 88 will usually be pumped toshore through a submarine pipeline, will be compressed and stored inanother unshown tank in storage structure 32, will be reinjected intosubsurface formations, and/or will be flared.

The invention we claim is:

l. A system for producing and storing fluid minerals underlying a bodyof water, comprising:

a. a floating storage structure having i. a ship-shaped hull,

ii. a fluid storage tank in said hull, and

iii. a. swivel assembly positioned along the longitudinal center lineand in the forward 20 percent of the length of the structure, the swivelassembly including a stationary core journaled in a vertical well in thefloating storage structure;

b. anchoring means secured to said stationary core of said swivelassembly and extending to the floor of the body of water, therebypermitting said floating storage structure to rotate freely through 360about a substantially vertical axis in response to wind and waterforces;

c. a supply flowline extending up through the body of water;

d. a separator unit having an inlet and at least one outlet, saidseparator unit mounted in said stationary core of said swivel assemblyand having said inlet connected to said supply flowline, said separatorunit being ada ted to separate the fluid flowing through said supplyowlme Patent No. 3 ,590,407 D t d July 6, 1971 Inventor(s) VintilaBratianu and Lewis A. Rupp It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Title page, second column, --Drude Faulconer-- should be included with"Attorneys".

Column 1, line 14, "'to wind" should be deleted and --atshould beinserted;

line 55, "OF" should be -of--;

line 75, "TAlley" should be --Talley-- Column 3 line 12, "an" should be--and--;

line 24, "raising" should be --rising--;

line 39, "man" should be -main- Column 4, Claim 1, line 34, "reduced"should be --reduce--;

line 39, "outline" should be --outlet-- Signed and sealed this 21st dayof December 1971.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCRALK Attesting Officer ActingCommissioner of Patents

1. A system for producing and storing fluid minerals underlying a bodyof water, comprising: a. a floating storage structure having i. aship-shaped hull, ii. a fluid storage tank in said hull, and iii. aswivel assembly positioned along the longitudinal center line and in theforward 20 percent of the length of the structure, the swivel assemblyincluding a stationary core journaled in a vertical well in the floatingstorage structure; b. anchoring means secured to said stationary core ofsaid swivel assembly and extending to the floor of the body of water,thereby permitting said floating storage structure to rotate freelythrough 360* about a substantially vertical axis in response to wind andwater forces; c. a supply flowline extending up through the body ofwater; d. a separator unit having an inlet and at least one outlet, saidseparator unit mounted in said stationary core of said swivel assemblyand having said inlet connected to said supply flowline, said separatorunit being adapted to separate the fluid flowing through said supplyflowline into its liquid and gaseous components and reduced the pressurethereof; e. a storage flowline having one end in communication with saidstorage tank; and f. a swivel means rotatably coupling the other end ofsaid storage flowline with said at least one outline of said separatorunit.